A polyester film, especially a biaxially oriented film made of polyethylene terephthalate or polyethylene naphthalate is widely used as a material for magnetic tapes, ferromagnetic thin film tapes, photo films, package films, films for electronic parts, electric insulating films, films to be laminated on a metal plate, films to be put on the surface of a glass display and films for protecting various members because it has excellent mechanical properties, thermal resistance and chemical resistance.
The polyester film is now frequently used in various optical films. For example, it is used as a base film for prism lens sheets, touch panels and back lights which are members of a liquid crystal display, base film for anti-reflection films, electromagnetic shielding film for plasma displays, base film for organic EL displays or base film for preventing the explosion of a display. The base films for use in the above optical films need to have excellent transparency. Further, they need to have excellent adhesion to a prism lens layer, hard coat, pressure sensitive adhesive, anti-reflection treatment and sputter layer.
Since the base films for optical use need to have transparency, the amount of a filler generally added must be minimized. To improve the bonding force of an adhesive layer, a resin having a low glass transition point is used in the adhesive layer. As the film for optical use contains a minimum amount of a filler or does not contain a filler at all, the surface of the film is flat. Further, as a resin having a low glass transition point is used in the adhesive layer, when the film is rolled or overlapped with another film, blocking occurs, whereby these films do not slide over each other, resulting in reduced handling ease or a cracked surface in the film forming step or processing step. Although lubricity and scratch resistance can be improved by adding fine particles to the adhesive layer to solve these problems, the haze of the adhesive layer may increase and transparency may lower. Further, when the base film is highly transparent, a small change in the thickness of an adhesive coating layer becomes a coating defect, thereby deteriorating the appearance of the film. To improve scratch resistance and lubricity, the use of fine particles having a particle diameter larger than 200 nm is proposed (JP-A 2001-96696 and JP-A 2000-229395). However, these technologies have a problem that the film is scratched by its contact with a hard object such as a metal roll due to the fall of fine particles or the low scratch resistance of the film.
It is proposed that an adhesive layer such as a polyester resin, acrylic resin or urethane resin adhesive layer is formed on the surface of a polyester film because a biaxially oriented polyester film generally has low adhesion to another material, for example, a prism lens layer or hard coat essentially composed of an acrylic resin (JP-A 10-119215 and JP-A 2000-246855). However, a biaxially oriented polyester film having an adhesive layer made of one of these resins may be unsatisfactory in bonding force. For example, a CRT film is unsatisfactory in adhesion to a pressure sensitive adhesive layer and therefore has low general applicability though it is satisfactory in adhesion to a hard coat layer on the opposite side.